Field of the Invention
This invention relates to scanners for creating successive sweeps of a spot of electromagnetic radiation along the same linear scan path at high frequency. Either the target at which the beam is directed is moved in a direction perpendicular to the linear scan path or the scanned beam is moved in such perpendicular direction, in order to create a raster.
Scanners may be used to write, in which case information is imposed on the beam usually prior to the creation of the successive sweeps of the beam. Scanners may also be used for reading, in which case the beam impining on the target has a constant intensity and the light reflected or transitted by the target being read is monitored to create a signal representative of the information on the scanned target.
Rotating scanners are known which include a device including a plurality of planar mirrors disposed uniformly around a cylinder. The planes of the mirrors are parallel to the axis of the cylinder and the radii through the centers of the mirrors are at uniform angular spacings around the axis. A motor rotates the device at high speed. A stationary beam of light is directed at the device which reflects the incident beam and creates successive sweeps of the light beam each along the same linear path. Such a mirror-containing device has been termed a polygon.
Scanners are also known in which a diffractive device is used instead of the reflective device for deflecting the beam. Such a diffractive device may include a disc of light-transmissive or reflective material on which are created a plurality of identical facets. Each facet contains a diffraction grating. The lines of the grating may be perpendicular to a radius bisecting the facet or they may be parallel to such a radius. The two types are termed tangential and "radial", respectively. The discs bearing the diffractive facets have come to be termed hologons.
It is known that in order to have high resolution of reading or writing information on the target, the effective portion of the beam should have a very small cross-sectional area upon impingement on the target. A focussing lens may be placed between the rotating scanning device and the target in order to focus the beam to a spot of the requirred small area. It is known that the effective diameter of the spot is inversely proportional to the diameter of the beam incident on the rotating device, i.e., the polygon or the hologon, hence a small spot requires a large beam on the rotating device.
The beam is only useful for reading or writing an image when it is wholly on a single facet of the hologon. When the beam is incident on two facets, two diffracted beams are created and each one is truncated thereby distorting the spots created on the target by these two beams. Also, one diffracted beam sweeps the end portion of a line and the other sweeps the beginning portion of a new line. At such times the system is not useful if writing an image because the same information would be written in two locations. If the system is reading an image, different information, gathered from different locations, would be read simultaneously. The duty cycle of a scanning system is defined as the ratio of active scan time to total scan time. Active scan time is defined as the time during which the beam lies wholly within one facet of the hologen. Total scan time is the time for the beam to pass a similar point on each of two successive facets. When designing scanning apparatus, a relatively small active scan time can be compensated for by increasing the data rate. However, such increase has its price and its limit. Evidently, the duty cycle of a hologon increases as the chordal dimension of the beam, where it is incident on the hologon, decreases. Thus, a high duty cycle practically requires a small chordal dimension of the beam at the hologon whereas high resolution requires that the beam have a large diameter at the hologon.
There is often a desire to further increase the speed of writing or reading. Thus, whether or not there is freedom to increase the information density, there is a desire to increase the number of scan lines produced in unit time. The number of scan lines produced in unit time is equal to the number of facets passed through the beam in unit time. Such number of facets is the product of the rotational speed of the hologon and the number of facets on the hologon. Thus, the reading or writing speed can be increased by increasing the number of facets on the hologon and by increasing the rotational speed of the hologon. Each manner of achieving higher speed has its problems. The stresses on the hologon increase with increasing speed and the power needed to drive the hologon increases rapidly with speed unless the undesirable step of putting a vacuum around the hologon is taken. If the number of facets is increased while keeping the hologon diameter and the beam shape constant, the complexity of making the hologon increases and the duty cycle decreases. In order to maintain the same duty cycle with an increased number of facets, the hologon diameter must be increased but such diameter increase greatly increases the stresses on the hologon and the power needed for the hologon drive. It is known that the power needed to drive a disc goes up approximately as the fifth power of the radius. The chordal dimension of the beam where incident on the hologon could be reduced in order to maintain the duty cycle but this would tend to adversely affect resolution of the system because the light distribution in the spot on the target is adversely affected.
The shape and orientation of the light spot on the target is also important. As stated above, for high resolution it must be small. However, it is advantageous that its cross-sectional shape be non-circular, it being larger in a direction transverse to the scan direction than it is in the scan direction. This is because the act of reading or writing one pixel of information is not instantaneous--it takes finite time. In that time the spot travels a finite distance. Thus, to read or write a circular pixel with high resolution the spot should be narrower in the direction of travel than it is in the direction transverse thereto.
Thus, it will be recognized that in creating a scanning apparatus there are many interrelated factors which have to be considered, for example, reading or writing speed, speed capability of the electronics in the read or write circuit, resolution, motor power, hologon cost, whether to put the hologon in a vacuum, and duty cycle. Many of these make conflicting demands on the shape and area of the beam.
It is an object of the present invention to provide a scanning apparatus with high resolution, high speed and acceptable motor power requirements.